This invention relates to measurement systems and methods and, more particularly, to measurement systems and methods for determining component particle concentrations in a liquid containing a mixture of relatively large particles and relatively small particles, utilizing light scattering techniques. The invention is particularly useful for determining fat and casein concentration in dairy products, but is not limited to such use.
It is frequently desirable to determine particle concentration in a liquid. Such measurements may be utilized in process control, research, and the like. In some cases, the liquid may contain particles of one type having one size distribution. In other cases, the liquid may contain particles of two or more types, each having a characteristic size distribution. In the latter case, determining particle concentrations is difficult because of interaction between measurements. An example of a liquid having two types of particles with different size distributions is milk and other dairy products.
Raw milk prices are determined by the fat content of the milk. In most retail markets, dairy processors standardize milk to various levels of fat, e.g., 1%, 2% and 3.25% in the United States. Also in cheese production, quality and yield can be optimized by standardizing the ratio of casein to fat in the milk used to make the cheese. Consequently, there is a need for an effective means of measuring fat and casein concentrations in milk. Standardization requires a robust measurement suitable for operation on the dairy production floor for process control applications.
Several automated techniques have been developed for measuring fat concentration in milk. Currently, measurement of the optical turbidity of milk provides the most accurate and stable instrument-based fat measuring technique. Simple broadband light attenuation measuring detectors are used. The milk sample is homogenized and is diluted with a high pH diluent, such as sodium hydroxide, to dissolve calcium caseinate from the milk, so that it does not interfere with the fat measurement. There are no accepted simple instrumentation methods for measuring casein concentration in milk.
Light scattering is a known technique for characterizing particles in a liquid. In a light scattering system, a liquid containing particles is passed through a sample cell having windows. A light beam is directed through the liquid, and light scattered by the particles in the liquid is analyzed to determine the characteristics of the particles. In one prior art system, the liquid sample is surrounded by an array of detectors which collect laser light scattered by the sample at different angles. In another prior art system, laser light scattered by the sample at a predetermined angle passes through an annular aperture and is focused on a photomultiplier. A light scattering system for molecular characterization is disclosed in U.S. Pat. No. 5,305,073 issued Apr. 19, 1994 to Ford, Jr.
All of the known prior art light scattering systems have had one or more disadvantages, including an inability to measure the concentration of relatively small particles in the presence of relatively large particles. Accordingly, there is a need for improved measurement systems and methods for determining component particle concentrations in a liquid, such as fat and casein concentrations in a dairy product.
According to a first aspect of the invention, apparatus is provided for measuring component concentration in a dairy product. The apparatus comprises a light source for generating a polarized light beam having a direction of polarization, a scattering cell, positioned in the light beam, for receiving a sample of the dairy product, a first light detector positioned at a first angle in a range of about 5xc2x0 to about 45xc2x0 with respect to the light beam and a second light detector positioned at a second angle in a range of about 130xc2x0 to about 160xc2x0 with respect to the light beam. The scattering cell has a first window and a second window, wherein the second window is farther from the light source than the first window, wherein a normal to an exterior surface of the second window is at or near Brewster""s angle with respect to the light beam and wherein the direction of polarization of the light beam is parallel to a plane defined by the light beam and the normal to the exterior surface of the second window. The first light detector detects scattered light from a first component of the dairy product having relatively large particle sizes and generates a first detector signal that is representative of concentration of the first component in the sample of the dairy product. The second light detector detects scattered light from a second component of the dairy product having a relatively small particle sizes and generates a second detector signal that is representative of concentration of the second component in the sample of the dairy product. The first component may comprise fat, and the second component may comprise casein.
In a preferred embodiment, the first angle is about 40xc2x0 and the second angle is about 140xc2x0. Because of refraction effects in the first and second windows, the actual scattering angles differ from the laboratory angles which define the positions of the light detectors, as described below. The light source may comprise a laser and a polarizing device.
According to another aspect of the invention, a method is provided for measuring component concentration in a dairy product. The method comprises the steps of generating a polarized light beam having a direction of polarization, placing a sample of the dairy product in a scattering cell that is positioned in the light beam, detecting scattered light from a first component of the dairy product at a first angle in a range of about 5xc2x0 to about 45xc2x0 with respect to the light beam and generating a first detector signal, and detecting scattered light from a second component of the dairy product at a second angle in a range of about 130xc2x0 to about 160xc2x0 with respect to the light beam and generating a second detector signal. The scattering cell has a first window and a second window, wherein the light beam is incident on the second window after passing through the sample of the dairy product, wherein a normal to an exterior surface of the second window is at or near Brewster""s angle with respect to the light beam and wherein the direction of polarization of the light beam is parallel to a plane defined by the light beam and the normal to the exterior surface of the second window. The first detector signal is representative of concentration of the first component in the sample of the dairy product, and the second detector signal is representative of concentration of the second component in the sample of the dairy product.
According to a further aspect of the invention, apparatus is provided for measuring component concentration in a liquid containing relatively large particles and relatively small particles. The apparatus comprises a light source for generating a polarized light beam having a direction of polarization, a scattering cell, positioned in the light beam, for receiving a sample of the liquid, a first light detector positioned at a first angle in a range of about 5xc2x0 to about 45xc2x0 with respect to the light beam, and a second light detector positioned at a second angle in a range of about 130xc2x0 to about 160xc2x0 with respect to the light beam. The scattering cell has a first window and a second window, wherein the second window is farther from the light source than the first window, wherein a normal to an exterior surface of the second window is at or near Brewster""s angle with respect to the light beam and wherein the direction of polarization of the light beam is parallel to a plane defined by the light beam and the normal to the exterior surface of the second window. The first light detector detects scattered light from a first component of the liquid having relatively large particle sizes and generates a first detector signal that is representative of concentration of the first component in the sample of the liquid. The second light detector detects scattered light from a second component of the liquid having relatively small particle sizes and generates a second detector signal that is representative of concentration of the second component in the sample of the liquid.